Redox active bio-ionic liquid electrolyte for high energy density Zn-ion capacitor

Zinc-ion capacitors are attracting significant interest due to their safety, recyclability, and high theoretical capacity (820 mAh g⁻¹). Most studies on Zn-ion capacitors focus on aqueous electrolytes, but these limit the electrochemical window and often lead to dendrite formation. In contrast, aprotic ionic liquid electrolytes extend the electrochemical window but suffer from slow diffusion kinetics of Zn species, which can reduce power density. In this work, we introduce a sustainable and biocompatible redox electrolyte based on bio-ionic liquids (Choline acetate and Choline iodide) for Zn-graphene capacitors. This electrolyte results in a high storage capacity of 350 F g⁻¹ at a current density of 0.5 A g⁻¹. Electrochemical tests, in situ Atomic Force Microscopy (AFM), and Density Functional Theory (DFT) studies reveal that choline in the electrolyte interacts with graphene, altering its local electronic structure and enhancing its capacity. The presence of choline iodide further improves the capacity through a redox reaction on the graphene surface. Stability tests at 3 A g⁻¹ show an initial capacity of 160 F g⁻¹, which decreases to 130 F g⁻¹ after 5000 cycles, yielding a capacity retention of 81.5 %. This study paves the way for the development of biocompatible hybrid capacitors for a range of applications.